Since Ogston first proposed a model of connective tissue consisting of "a relatively coarse fibrous collagen network" that balances the osmotic pressure of a "molecular network of polysaccharide fibers trapped within it", surprisingly little work has been done to use it to study the behavior of cartilage. (Ogston, A. G. (1970) in Chemistry and Molecular Biology of the Intracellular Matrix: The biological functions of the glycosaminoglycans Eds. (13alazs, E. A., Eds.), pp. 1231-1240, Academic Press, London).
Subsequent studies have been performed to measure the proteoglycan (PG) osmotic pressure, .pi..sub.PG, in normal and osteoarthritic (OA) cartilage specimens, yet, no comparable quantitative methods have been developed to characterize the integrity of the collagen network per se, such as its ability to "restrain" the PGs from swelling. See, Maroudas, A., Bayliss, M. T., and Venn, M. F. (1980) Ann Rheum Dis 39, 514-23; Grushko, G., Schneiderman, R., and Maroudas, A. (1989) Connective Tissue Research 19, 149-176; and Maroudas, A., Ziv, I., Weisman, N., and Venn, M. (1985) Biorheology 22, 159-69, which are herein incorporated by reference. Consequently, no methods are available to characterize the PG and collagen network phases in situ.
Being able to characterize the PGs and the collagen network together, however, is particularly important in understanding the etiology of OA. Although there has been indirect evidence presented that suggests that the collagen network loses mechanical integrity early in OA whereas PG content and composition may not change appreciably, it has not possible to demonstrate this definitively since heretofore no methodology for characterizing both the state of the PGs and of the collagen network phases in situ in the same tissue specimen has been developed. See, Maroudas, A. and Venn, M. (1977) Ann Rheum Dis 36, 399-406. Maroudas, A. (1976) Nature 260, 808-9. Maroudas, A., Evans, H., and Almeida, L. (1973) Ann. Rheum. Dis. 32, 1-9.
More generally, experimental and theoretical tools to determine collagen network integrity are also needed to understand the functional consequences of a) endogenous structural changes in cartilage (e.g., that occur normally in normal and abnormal development, aging, degeneration, and disease), b) exogenous changes (e.g., following the addition of biochemical agents such as proteinases, or resulting from genetic manipulations), and c) inherent differences between cartilage tissues (e.g., between species; as well as between different joints, different locations on the same joint, and young and old individuals of the same species). In addition, there is a need to understand how environmental (chemical, mechanical, or electrical) stresses affect connective tissue structure and function. These stresses may be endogenous (e.g., occurring during locomotion), or exogenous (e.g., applied externally to tissue cultures in vitro or during a clinical evaluation).
Further, it is becoming increasingly important to evaluate the growth and viability of the collagen network within tissue-engineered connective tissues, both in vitro (e.g., in drug efficacy studies), and in vivo prior to and following their implantation. Without a quantitative measure of collagen network integrity together with the swelling characteristics of the PGs, the relationship between collagen network structure and tissue function cannot be established.
There is, therefore, a need for a quantitative methodology for determining the mechanical integrity of the collagen network in cartilage and of other extracellular matrices.